An amorphous material is a solid in which the atoms exhibit no long range order and are bound to each other irregularly, as opposed to a crystalline material, which has a regular repeating internal structure. An example of an amorphous material is ordinary window glass, which is formed when molten silicate with high viscosity is cooled, without allowing a regular crystal lattice to form. The amorphous state of the glass results in various useful optical properties, such as its transparency. The presence of various contaminants and impurities may have a significant influence on the final properties of the amorphous material (e.g., its color, transparency, melting point).
Group-III metals of the periodic table (i.e., aluminum, gallium and indium) can form nitrides, i.e., aluminum nitride (AlN), gallium nitride (GaN) and indium nitride (InN). Group-III metal nitrides are semiconductors having various energy gaps (between two adjacent allowable bands), e.g., a narrow gap of 0.7 eV for InN, an intermediate gap of 3.4 eV for GaN, and a wide gap of 6.2 eV for AlN. Solid group-III metal nitrides have an ordered crystalline structure, giving them advantageous chemical and physical properties, such that electronic devices made from group-III metal nitrides can operate at conditions of high temperature, high power and high frequency. Electronic devices made from group-III metal nitrides may emit or absorb electromagnetic radiation having wavelengths ranging from the UV region to the IR region of the spectrum, which is particularly relevant for constructing light emitting diodes (LED), solid-state lights and the like.
To be used in various technological applications, the group-III metal nitride crystals may be in the form of a free-standing wafer or a thin film, attached to an arbitrary platform of conducting, semi conducting, or dielectric nature. For other uses, group III metal nitrides may be in the form of a free standing bulk crystal. For industrial applications, group III metal nitride crystals of large size (i.e., substantially 25 mm or larger) are required. However, crystals of large size, having a low defect density, are difficult to manufacture.
Group-III metal nitride crystals are not found naturally and are artificially produced as thin films on a crystalline substrate, by methods known in the art. Among the group III metal nitrides, gallium nitride can be produced using hetero epitaxy, wherein the substrate used as a hetero epitaxial template can be, for example, a single crystalline wafer of sapphire (Al2O3), on which a layer of GaN is deposited. Alternatively, a silicon carbide (SiC) wafer may be used as a substrate. However, due to the difference in lattice parameters between the substrate and the GaN layer, various crystal defects may appear in the GaN crystal.
Other known methods for growing group-III metal nitride crystals employ a metallic melt, typically of the group-III metal. Nitrogen is supplied to the melt and chemically reacts with the group III metal in the melt, thereby enabling crystal growth. Such methods are often expensive, and the crystal dimensions achieved, as well as the quantity of crystals produced, are typically small for industrial applications. Group-III metal nitride crystals, manufactured according to methods known in the art usually have crystal defects therein, such as dislocations, misorientations, vacancies, interstitial atoms, impurities, and grain boundaries. In particular, none of the above mentioned methods are used to produce GaN crystal sheets of large dimensions, having a low defect density of less than 103 defects per centimeter squared.
Amorphous group-III metal nitrides have certain useful optical properties, making them possible candidates for a variety of applications, such as solar batteries and full color displays.
Techniques for preparation of material films include: thin-film deposition processes (e.g., sputter deposition and chemical vapor deposition), Molecular Beam Epitaxy (MBE), and ion implantation. Thin-film deposition involves depositing a thin film onto a substrate, or on previously deposited layers on the substrate.
MBE is a method for epitaxially growing layers of materials onto a substrate, by slowly directing a beam of particles toward the surface of the substrate. MBE generally requires a high vacuum in the reaction chamber, in order to avoid impurities in the epitaxially formed material. The epitaxy deposition rate in MBE is considered slow, relative to other deposition techniques.
Sputter deposition is one type of thin film deposition technique. The atoms in a solid target material are ejected into a gas phase by ion bombardment. Each collision knocks off additional atoms, where the number of ejected atoms per incident ion (i.e., the sputter yield) is dependent on several factors, such as the energy of the incident ions, the respective masses of the ions and atoms, and the binding energy of the atoms in the solid. The ions are provided by a plasma, usually of a noble gas (e.g., argon). The ejected atoms are not in their thermodynamic equilibrium state, and tend to deposit on all surfaces in the vacuum chamber. Therefore a substrate in the chamber will end up being coated with a thin film having the same composition of the target material. The target can be kept at a relatively low temperature during sputter deposition, since no evaporation is involved. In reactive sputtering, the plasma gas includes a small amount of a non-noble gas, such as oxygen or nitrogen, which reacts with the material after it is sputtered from the target, resulting in the deposited material being the product of the reaction, such as an oxide or nitride.
Chemical vapor deposition (CVD) is another type of thin film deposition, where the film is formed by a chemical reaction. The substrate is exposed to a mixture of gases, which reacts with the substrate surface to produce the desired deposit, which condenses on the substrate. CVD can be performed at medium to high temperature in a furnace, or in a CVD reactor in which the substrate is heated. Unwanted byproducts are usually also produced in the reaction, which are removed by gas flow through the reaction chamber. Plasma may be used to enhance the rates of chemical reaction. Metal-organic chemical vapor deposition (MOCVD) involves organo-metallic compounds as the reactants.
Ion implantation involves implanting ions of a first material in a second target material. The ions are electrostatically accelerated to a high energy, before impinging on the target material, such as on the surface of a substrate. The amount of material implanted, known as the dose, is the integral over time of the ion current. By controlling the dose and the energy, along with the applied temperature of the target, it is possible to change the crystal structure of the target surface in such a way that an amorphous layer is formed. The impinging ions break chemical bonds within the target material, and form new bonds which are unorganized and not in thermodynamic equilibrium, resulting in the target material becoming amorphous.
U.S. Pat. No. 5,976,398 to Yagi, entitled “Process for manufacturing semiconductor, apparatus for manufacturing semiconductor, and amorphous material”, is directed to an amorphous nitride III-V compound semiconductor, and an apparatus and process for its manufacture. The manufacturing process utilizes plasma-enhanced MOCVD. The semiconductor manufacturing apparatus includes a reactor, a first and second activation-supply portions, an exhaust pipe, a heater, and a substrate holder. The substrate holder holds a substrate inside the reactor, which is allowed to form a vacuum. Each activation-supply portion is composed of a pair of gas introducing pipes, a quartz pipe connected with the reactor, and a microwave waveguide (or alternatively, a radio frequency coil) for providing activation.
Plasma of a V group element (e.g., nitrogen plasma) is generated at the first activation-supply portion and introduced into the reactor. For example, N2 gas is introduced from the gas introducing pipe, and a microwave oscillator supplies microwaves to the microwave waveguide, which induces a discharge in the quartz pipe and activates the N2 gas. A metal organic compound containing a III group element (e.g., Al, Ga, In) is supplied by a gas introducing pipe of the first activation-supply portion. An auxiliary material (e.g., He, Ne, Ar, H2, Cl2, Fl2) is supplied by the gas introducing pipe of the second activation-supply portion. The auxiliary material (e.g., hydrogen plasma) reacts with an organic functioning group of the metal organic compound, including the III group element, to inactivate the organic functional group. The vaporized metallic organic compound and the plasma of the auxiliary material is added to the plasma of the V group element.
The heater heats the substrate to the appropriate temperature (e.g., from 200° C. to 400° C.). A film of amorphous material, containing the III group element and the V group element, is formed on the substrate. The film of the semiconductor compound contains the III group element and the V group element. For example, the amorphous material is hydrogenated amorphous gallium nitride. The amorphous material is suitable as an optical semiconductor for optoelectronic applications.
US Patent Application Pub. No. US 2002/0100910 to Kordesch, entitled “Band gap engineering of amorphous Al—Ga—N alloys”, is directed to an amorphous semiconductor alloy including aluminum and gallium, and a method for its production, which utilizes sputter deposition. A semiconductor substrate is positioned on an anode inside a reactive sputter deposition chamber. The sputter deposition chamber also includes a sputter target on a target cathode. The sputter deposition chamber is coupled with an RF source and a matching network. The sputter target contains aluminum and gallium (e.g., a single integrated target with both aluminum and gallium, a single target with an aluminum portion and a gallium portion, or discrete targets of aluminum and gallium). The sputter target may also contain indium. Nitrogen gas is introduced into the sputter deposition chamber. The sputter deposition chamber is operated to promote reaction of the aluminum and gallium of the sputter target with the nitrogen. The semiconductor substrate is maintained at a deposition temperature (e.g., between about 77K to about 300K), selected to ensure that the grown alloy is amorphous. The relative proportions of aluminum and gallium are selected such that the amorphous alloy will have a band gap between about 3 eV and about 6 eV. The amorphous alloy has the chemical formula: AlxGa1-xN. The amorphous alloy may be doped, such as with a rare earth luminescent center, for various photonics applications.